Predicting a life of a rolling bearing is important in selecting the type of the bearing and optimizing a bearing replacement timing. However, because of the complexity of rolling contact characteristics and a large number of parts related to fatigue, even if the same type of bearing is used under the same condition, the life of the rolling bearing is largely varied. Therefore, a method in which Weibull distribution is applied to a bearing life distribution state, and a value representative of the distribution is used has been proposed, and the method is still in use. In addition, the most frequently used life of the bearing is a basic rated life L10 represented by the following Expression (1) as a life in which 10% of the total number of bearings are damaged.
                    [                  Ex          .                                          ⁢          1                ]                                                                      L          10                =                              (                          C              P                        )                    p                                    (        1        )            
In this example, C is called a basic dynamic load rating and is a parameter indicating a dynamic load capacity of the bearing. In addition, P is an equivalent load to be applied to the bearing. An index p is 3 for ball bearings and 10/3 for roller bearings.
The basic rated life L0 has been used for a long period of time, but thereafter it becomes clear that the life of the bearing is affected by a fatigue limit load of the bearing, a lubrication condition, an operating environment, contaminated particles (iron powder or dust mixed in a lubricant) during operation, and a cleanliness at the time of installation. A correction factor aiso considering those influences has been proposed on the basis of a large number of test results, and a modified rated life Lnm represented by the following Expression (2), which is obtained by multiplying L10 by a coefficient a1 for calculation of the correction factor and any damage probability n %, has been proposed.
                    [                  Ex          .                                          ⁢          2                ]                                                                      L          nm                =                              a            1                    ⁢                                                    a                iso                            ⁡                              (                                  C                  P                                )                                      p                                              (        2        )            
In recent years, there has been a growing demand for optimizing the replacement timing of the rolling bearing to maximize product availability. As a method of meeting the demand, for example, Patent Literature 1 or Patent Literature 2 discloses a method of measuring a physical quantity relating to the rolling bearing at the time of actual operation and evaluating the fatigue state of the rolling bearing that changes from moment to moment, and a method of measuring a load at the time of actual operation, which is further required.
Patent Literature 1 discloses a tapered roller bearing having strain gauges which are spaced apart from each other in an axial direction, and disposed on a circumferential surface of an outer circumferential surface of the tapered roller on a sixed fixed to an inner circumferential surface of an outer ring, a substrate that is disposed along an axial direction at the center of the tapered roller, and a processing unit that is mounted on the substrate, and configured to measure the load applied to the tapered roller bearing.
In addition, Patent Literature 2 discloses a technique of storing basic data obtained by measuring a state of a diagnostic rolling bearing in advance, measuring a load to be applied to the bearing, which is measured by an acceleration sensor, detecting a deterioration state of a lubricant having a large influence on a life of the diagnostic rolling bearing based on a measured value and basic data, and evaluating a remaining life of the rolling bearing on-line on the basis of the detected result.